There’s one thing that’s always bothered us about The Walking Dead, the highly successful comic book and TV series. None of the characters ever invoke the Z word. But why? Is it because Rick, Darryl, and Michonne have never seen a George Romero movie? Or is it because the word “zombie” is protected under copyright law? It’s weird that the crew would coin a euphemism like “walker” for a reanimated corpse when a more specific word already exists.

Similarly, there’s an ongoing joke about zombies in Peter Clines’ latest zombocalypse adventure. The superheroes continually ask themselves: “What were the monsters called in George Romero’s movies?” Initially, none of them can remember. “They’re just dead things, right?” asks Madelyn Sorensen at one point. Which, incidentally, is kind of ironic since Madelyn is a zombie herself.

Finally, the heroes figure it out (probably because the joke was starting to get a little stale): the dead things walking around New Los Angeles are zombies. Even though these undead creatures are called “ex-humans” in this particular mythology, the heroes have enough awareness to identify a zombie as a zombie (unlike the vocabulary-challenged cast of The Walking Dead).

Back in 2010 when the first Ex-Heroes book was published, no one could have predicted that a fourth book (and a forthcoming fifth book) would ever appear on the shelves. In fact, we’re betting even the author himself didn’t know how long he could pump out these superhero/zombie novels. After reading Ex-Purgatory, however, we’re happy to discover that the series still has some gas left in the tank.

The novel starts off with all the Ex-heroes trapped in a prison they have no desire to escape from. It’s like they’re in a Star Trek holodeck—there are no zombies, no evil Homeland Security agents, no complications whatsoever. The Mighty Dragon is simply a janitor named George Harrison Bailey. That’s it.

But slowly the holodeck simulation starts to fizzle. George and his Ex-Men eventually realize they are living inside a ship in a bottle. How they escape from their Star Trek/Matrix prison is the true mystery of Ex-Purgatory. Superheroes, monsters, mecha, talking cars, presidential conspiracies, familial complications, and Hollywood inside jokes: this one’s got it all. And don’t forget about the zombies—or the ex-humans or the walkers (or whatever you want to call ’em).